The present invention relates to the field of mass storage devices. More particularly, this invention relates to an improved apparatus and method for writing servo information to the disc of a high density disc drive.
One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are a disc drive housing, a disc that is rotated, an actuator assembly that moves a transducer to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
To read and write data to the disc drive, the actuator assembly includes one or more arms that support the transducer over the disc surface. The actuator assembly is selectively positioned by a voice coil motor which pivots the actuator assembly about a pivot shaft secured to the drive housing. The disc is coupled to a motorized spindle which is also secured to the housing. During operation, the spindle provides rotational power to the disc. By controlling the voice coil motor, the actuator arms (and thus the transducers) can be positioned over any radial location along the rotating disc surface.
The transducer is typically placed on a small ceramic block, also referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (xe2x80x9cABSxe2x80x9d) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equalize so the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on portions of the storage disc referred to as tracks. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto the track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write to or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is often divided between several different tracks. While most storage discs utilize a multiplicity of concentric circular tracks, other discs have a continuous spiral forming a single track on one or both sides of the disc.
During manufacturing, servo information is encoded on the disc and subsequently used to accurately locate the transducer. The written servo information is used subsequently to locate the actuator assembly/transducer head at the required position on the disc surface and hold it very accurately in position during a read or write operation. The servo information is written or encoded onto the disc with a machine commonly referred to as a servo track writer (hereinafter STW). At the time the servo information is written, the disc drive is typically at the xe2x80x9chead disk assemblyxe2x80x9d (hereinafter HDA) stage. The HDA includes most of the mechanical drive components but does not typically include all the drive electronics. During the track writing process, the STW precisely locates the transducer heads relative to the disc surface and writes the servo information thereon. Accurate location of the transducer heads is necessary to ensure that the track definition remains concentric. If the servo track information is written eccentrically, the position of the transducer head during subsequent operation will require relatively large, constant radial adjustments in order to maintain placement over the track center. When the tracks are sufficiently eccentric, a significant portion of the disk surface must be allotted for track misregistration.
The offset between the actual head position and the track center is called the track misregistration. Track misregistration has two aspects, referred to as write-to-write track misregistration and write-to-read track misregistration. The write-to-write track misregistration is the misregistration between a recorded track and an adjacent track, which can result in track-to-track squeeze. The write-to-read track misregistration is the misregistration between the centerline of recorded track and the actual read head position. The misregistration is induced during servo track writing where any relative movement between the head and media in the radial direction will result in imperfect tracks written. The imperfect tracks written can cause adjacent tracks to position themselves closer than intended at certain locations on the media which gives rise to track squeeze. Also, such relative motion of the head to media is not synchronous to the rotation of the disc, i.e., spindle. Thus, the relative radial position of the head at the start of a track write will shift by the time it reaches the end of the track write. This leads to a discontinuity in the radial direction of the servo track. The squeeze and discontinuity contributes to what is known as written-in repeatable runout (STWRRO) in the servo track.
In order to ensure proper writing of servo information, STWs utilize an external, closed loop positioning system that precisely positions the transducer head during servo track writing. The positioning system comprises a contact member that engages the actuator assembly, a position indicator which indicates the position of the contact member, and a displacing mechanism which repositions the contact member based on feedback from the position indicator. To ensure accurate positioning, various position indicators are used (e.g., mechanical, capacitive, and optical transducers to name a few). The STW further includes the required circuitry for writing the servo information to the disc surface via the transducer heads.
While it is advantageous to maintain substantial concentricity during the servo track writing process, many factors adversely impact the STW""s ability to write servo information concentrically. As mentioned previously, servo track written repeatable runout occurs when relative motion occurs between the servo writing head and the media. One major contributor to the relative motion between the head and media is from the vibration of the spindle which is also known as the non-repeatable runout (NRRO). The NRRO is non-synchronous to the spindle rotation but is periodic. The major contributor to the NRRO is the cage rotation or ball train; thus, a periodic waveform is found. This wave will not be synchronized to the spindle but nevertheless repeats itself at intervals longer than one spindle rotation. In short, vibrations in the spindle or actuator components due to imperfect bearings is one major factor in producing non-repeatable track writing errors.
As a result of servo written non-repeatable runout, the overall track density achievable is reduced since there must be a xe2x80x9cbudgetxe2x80x9d for track misregistration. As a result, a track misregistration budget must be used to account for track misregistration or allow track squeeze to occur. Many factors contribute to the track misregistration problem, but it will be appreciated tat reducing servo track written repeatable runout will allow the reduce the overall track misregistration budget and allow for an increase in overall track density and an increase in disc drive capacity.
As demand for higher capacity drives grows, manufacturers are constantly seeking to increase drive capacity by increasing track density. That is, by increasing the density or xe2x80x9ctracks per inchxe2x80x9d (TPI), a greater number of discreet tracks can be encoded on a given disc surface. However, higher track density requires more efficient use of the disc surface. Accordingly, track misregistration due to eccentricities in track formation must be minimized in order to maximize TPI (and thus disc capacity).
Accordingly, what is needed is an apparatus and method for use with an STW that minimizes relative motion between the head of the servo writer and the disc. This will, in turn, reduce the servo track written repeatable runout and track squeeze which will in turn reduce the overall track misregistration (TMR) budget so that tracks may be more closely spaced. In particular, what is needed is a way to reduce non-repeatable run out caused by motion between the head and a given disk surface. What is also needed is a method wad apparatus which can reduce non-repeatable run out using Current manufacturing methods.
The present invention is directed to reducing these problems, especially the non-repeatable track writing errors due to imperfect bearings. A servo track writer for writing servo information to a head disc assembly includes a mounting fixture for mounting a head disc assembly. The head disc assembly includes a disc attached to a base with a spindle bearing, and a rotary actuator adjacent the disc. The actuator includes a read/write head for selectively magnetizing the disc. The mounting fixture provides a mechanical reference for the servo track writer as servo information is written to the head disc assembly. The servo track writer also includes a pusher block assembly mounted to the mounting fixture proximate the actuator of the head disc assembly. The pusher block assembly mechanically positions the actuator. The servo track writer also includes a position control system for controlling the position of the actuator. The position control system further includes a motor connected to the pusher block assembly, and a controller for outputting control signals to the motor in order to adjust the position of the actuator with respect to the disc. The controller synchronizes the motion of the actuator to a cage frequency of the spindle bearing. The position control system may further have a sensor for monitoring the cage frequency of the spindle bearing of the head disc assembly.
The controller selects the time to start servo writing in response to the cage frequency of the spindle bearing. The position control system of the servo track writer may also include a detector for determining the position of the actuator and outputting control signals to the motor in order to adjust the position of the actuator in response to the detected position of the actuator and a desired position of the actuator.
The controller for outputting control signals to the motor may synchronize to the spindle rotation frequency in addition to the spindle bearing cage frequency. The servo track writer may also have a sensor for monitoring the cage frequency of the spindle bearing of the head disc assembly. The servo track writer which synchs to both the spindle rotation frequency and the spindle bearing cage frequency may also have a sensor for monitoring the cage frequency of the spindle bearing of the head disc assembly. The controller selects the time to start servo writing in response to the cage frequency of the spindle bearing. The servo track writer further includes a detector for determining the position of the actuator and outputting control signals to the motor in order to adjust the position of the actuator in response to the detected position of the actuator and a desired position of the actuator.
Also disclosed is a method for writing servo patterns to a disc drive which includes at least one disc mounted on a spindle with a spindle bearing, the at least one disc mounted for rotation about an axis. The disc drive also includes an actuator for moving each of at least one transducer relative to an associated at least one disc. The transducer reads and writes information to the associated disc. The servo writer includes a linkage for controllably moving the actuator during servo writing. The method for writing servo patterns includes collecting spindle run out data, determining a spindle bearing cage frequency from the collected data, and moving the linkage of the servo writer to synchronize movement of the actuator to the spindle bearing cage frequency. Determining a spindle bearing cage frequency further includes calculating the spindle bearing cage frequency. The linkage of the servo writer is moved to synchronize movement of the actuator to the spindle bearing cage frequency. This includes generating run out at the spindle bearing cage frequency, and calculating the phase of the spindle bearing cage frequency. The movement of the linkage of the servo writer may also include timing the initial movement of the linkage of the servo writer to start the servo write process based on the phase of the spindle bearing cage frequency. The timing is based on measuring the phase at the spindle cage frequency or upon predicting the phase at the spindle cage frequency. The timing of the initial movement of the linkage of the servo writer to start the servo write process based on the phase of the spindle bearing cage is selected when the spindle bearing cage frequency is at the minimum.
The method for writing servo patterns to a disc drive may further include the step of determining a spindle frequency, and using the spindle frequency in addition to the bearing cage frequency to synchronize movement of the actuator the spindle bearing cage frequency aid to synchronize movement to the spindle frequency. Determining a spindle bearing cage frequency from the collected data further includes calculating the spindle bearing cage frequency.
Advantageously, the method and apparatus of the present invention produces disc drives with higher storage capacity than those drives produced by other servo writing methods/apparatuses. In particular, the instant invention permits precise, concentric writing of servo information to the disc. This reduces the servo track written repeatable run out and reduces the overall TMR budget so that tracks may be more closely spaced. Allowing tracks to be more closely spaced allows for the higher storage capacity of the disc drive. Advantageously, the method and apparatus reduce non-repeatable run out using current manufacturing apparatus and methods.